The Government has failed to make a full business case for the roll out of Universal Credit (UC), MPs have said.
People are being expected to take it on good faith that the controversial welfare shake-up will deliver, according to the chairman of the Commons Work and Pensions Committee Frank Field.
And the roll out of UC, beset by delays, still has some of its toughest challenges to deal with, a report by the committee states.
After examining internal project assessment reviews of the UC programmes's finance and delivery by the Infrastructure and Projects Authority (IPA), the committee expressed concerns about the situation.
The IPA called for the "industrialisation" of the UC programme for "complex cases and vulnerable customers" in order to realise efficiency savings, while "evidence on employment benefits has yet to be produced", MPs said.
The report warned that some of the biggest challenges facing UC, such as delivering an automated online service on a national scale, lie ahead.
Mr Field said: "Perhaps the most damning point that emerges from any assessment of the Government's progress on Universal Credit is that in the eighth year of the programme, the department itself has yet to produce the full business case for its own mega reform.
"The programme managers appear to expect us, the public, and the minister responsible to take it on faith that UC will deliver the much improved employment outcomes they claim for the vast range of people - disabled, single parents, carers, the self-employed - who will claim UC.
"At the moment, they are relying on the simplest cases - single, unemployed claimants with no children. They have produced no evidence to back up the key, central economic assumption of the biggest reform to our welfare system in 50 years. William Beveridge will be rolling in his grave.
"The reviews, which barely mention claimants, are also shot through with management gobbledegook.
"Were I the minister in charge, I would have either rejected or ignored much of it entirely as totally incomprehensible.
"This major reform would surely have been served better by a much more transparent approach."
The report raised questions about the Department for Work and Pensions' handling of the roll out, stating: "The department has consistently struggled to convince the IPA that UC can be scaled up as planned. The department must balance the considerable costs of further delays against the costs of pressing ahead.
"Chronic delays and revisions to the roll out are a recurrent theme through the reports, persisting up to the present day."
The committee said the IPA was "consistently critical of the department's failure to set clear criteria for proceeding to the next stage of the UC roll out."
MPs also said it was "very surprising" that UC had not been subject to a project assessment review since October 2015.
Labour's shadow work and pensions secretary Debbie Abrahams said: "After years of Tory Universal Credit chaos, this report proves once again that Universal Credit is not fit for purpose.
"It is completely unacceptable that the Government has not provided any evidence to back up their repeated assertion that Universal Credit will help people into work, a key principle of the programme."
A DWP spokesman said the department had been commended for improving the UC programme and 'substantial achievements' had been delivered since 2013.
A statement added: "Universal Credit is the biggest modernisation of the welfare system in a generation and continues to be delivered in a safe and secure way.
"The Infrastructure and Projects Authority has independently supported our test and learn approach, the improvements we continue to make and the recent expansion of the programme.
"People on Universal Credit are moving into work faster than those on the old system, staying in work longer and keeping more of their money. And in the Budget we announced a £1.5bn package of additional support for people as they move to the new system."